I always found it disappointing that desktop environments usually only support a single mouse pointer and a single focused window / insertion point for keyboard input.
Unixy systems might support multihead where devices could be routed to separate desktops, but I'm thinking more like
If you had a very large 4k tv hooked up two a computer with two keyboards and two mice/trackpads, and did pair programming with a colleague or a child, able to edit separate windows/tabs or the same one simultaneously.
I guess the modern equivalent is you use your separate devices but they talk to a shared multiuser editor server *shrug* close enough :D
@brion onme of my monitors allegedly supports multiple inputs - more then one computer - but that is not what you seem to be talking about. I also sometimes wish I could attach two mice or two keyboards and have two cursors separately on the same computer and monitor.
I think this would not require to many things - kernel and drivers I think would support this, they can already tell from which device the signal came (or at least I think so), so it would be higher up in the os, and in the DE, if used.